fix: ensure distinct property is respected cloning query builder (#4843)

* fix #4842: ensure select distinct property is respected when cloning query builder.

* Remove unused import statement.

* Node v12? added reflect-metadata import.

* Address lint issue.

src/query-builder/QueryExpressionMap.ts

@@ -378,6 +378,7 @@ export class QueryExpressionMap {
         const map = new QueryExpressionMap(this.connection);
         map.queryType = this.queryType;
         map.selects = this.selects.map(select => select);
+        map.selectDistinct = this.selectDistinct;
         this.aliases.forEach(alias => map.aliases.push(new Alias(alias)));
         map.mainAlias = this.mainAlias;
         map.valuesSet = this.valuesSet;

test/github-issues/4842/entity/Post.ts

@@ -0,0 +1,12 @@
+import {Column, Entity, PrimaryGeneratedColumn} from "../../../../src/index";
+
+@Entity()
+export class Post {
+
+    @PrimaryGeneratedColumn()
+    id: number;
+
+    @Column()
+    title: string;
+
+}

test/github-issues/4842/issue-4842.ts

@@ -0,0 +1,23 @@
+import "reflect-metadata";
+import {expect} from "chai";
+import {closeTestingConnections, createTestingConnections, reloadTestingDatabases} from "../../utils/test-utils";
+import {Connection} from "../../../src/connection/Connection";
+import {Post} from "./entity/Post";
+
+describe("github issues > #4842 QueryExpressionMap doesn't clone distinct property", () => {
+    let connections: Connection[];
+    before(async () => connections = await createTestingConnections({
+        entities: [__dirname + "/entity/*{.js,.ts}"],
+    }));
+    beforeEach(() => reloadTestingDatabases(connections));
+    after(() => closeTestingConnections(connections));
+
+    it("should contain correct distinct value after query builder is cloned", () => Promise.all(connections.map(async connection => {
+      const query = connection.manager.createQueryBuilder(Post, "post")
+          .distinct()
+          .disableEscaping();
+        const sqlWithDistinct = query.getSql();
+
+        expect(query.clone().getSql()).to.equal(sqlWithDistinct);
+    })));
+});